In mosfet, the gate is always referenced with respect to the source. If your gate is at a higher voltage than the source, then Vgs is +ve. If gate is at a lower voltage than the source then Vgs is -ve.

Usually for Pmosfet biasing, the source is connected to Vcc. So if you connect the gate to Vcc, then mosfet is Off because Vgs = Vg-Vs = Vcc-Vcc=0

if you pull gate to Gnd, then Vgs = Vg-Vs = 0-Vcc = -Vcc. Then the mosfet is ON.